As the developing method of the electro-photographic process, there is a method to rotate the developing sleeve and the photoreceptor in the same direction and a method to rotate the above in the opposite direction. The prior is called a “with method” and the latter is called a counter method. Among the above, according to the counter method, as shown in FIG. 18, due to a developing sleeve b rotating in a direction opposite of a photoreceptor a, the toner on an image front end c1 side is pushed swept to a rear end c2 side by a tip of the carrier. This causes worsening of the image quality called blur in which density of the image is light at the front end c1 and sweeping in which the density becomes dark at the rear end c2. According to the with method, the above occurs with the front end and the rear end of the image being opposite.
Methods to improve the uneven distribution of toner by image processing are proposed (for example, see Japanese Patent Application Laid-Open Publication No. 2009-58765, Japanese Patent Application Laid-Open Publication No. H11-196277, Japanese Patent Application Laid-Open Publication No. H11-346315). For example, the region in which variation in density occurs due to sweeping of toner is estimated based on edges of characters and shapes detected by analyzing the image data and correction is performed to increase the pixel value in the region where decrease of density occurs and to decrease the pixel value in the region where increase in density occurs.
There is also a method to detect the variation of density of the image with the sensor and to adjust the developing condition according to the detected density variation (for example, see Japanese Patent Application Laid-Open Publication No. H7-175367, Japanese Patent Application Laid-Open Publication No. H10-142856).
According to the above cited documents which correct the image data, the edge of the image such as characters and shapes are detected, and from the edge direction, contrast, etc., the image region to be corrected and the correction value are determined. Since it is necessary to observe the image region near the edge to be detected and the image region to be corrected at the same time, at least the image data including both image regions are deployed in the memory. The image region held in the memory for such observation is typically called an observation window.
For example, in order to detect the edge of the image which is the start of sweeping, it is necessary to observe the pixel value of both the inside and the outside of the edge at the same time, and the region with 3 to 5 pixels is necessary. The sweeping may occur 0.6 mm in the sub-scanning direction in a wide range, and when the resolution is 600 dpi, 0.6 mm corresponds to 15 pixels. As shown in FIG. 19, when the region with 5 pixels is observed to detect the edge, and the region with 15 pixels from the edge is corrected, the observation window with 17 pixels including both regions is necessary. That is, in order to correct blur and sweeping, it is necessary to obtain a memory with a size including 17 pixels (N=17) in the sub-scanning direction.
As the observation window becomes larger, the necessary memory capacity increases, the circuit scale is enlarged, and conventionally, a means to decrease the memory capacity in order to suppress the circuit scale was not considered. According to Japanese Patent Application Laid-Open Publication No. 2009-58765, Japanese Patent Application Laid-Open Publication No. H11-196277, and Japanese Patent Application Laid-Open Publication No. H11-346315, rich memory resources such as the work memory and the page memory used by the CPU are assumed and there is no consideration in optimizing the memory capacity to be used to a minimum amount.
According to the image forming apparatus such as a copier or a printer, a plurality of types of image processes are connected by a pipeline process in order to enhance image quality from the viewpoint of simplifying the configuration of the apparatus and enhancing performance (throughput). The pipeline process connects the plurality of process elements in a series, and a certain process cannot reuse the resources used in the previous process (circuit, processing apparatus, memory, etc.). When the memory corresponding to the above-described large observation window is mounted in such processing circuit of the pipeline process, the circuit scale becomes massive and this leads to increase in costs.